This invention relates in general to apparatus for controlling temperature and, more particularly, to apparatus for controlling the temperature of a resistive electrical heater to reduce flicker.
Photothermography is an established imaging technology. In photothermography, a photosensitive media is exposed to radiation to create a latent image which can then be thermally processed to develop the latent image. Devices and methods for implementing this thermal development process are generally known and include contacting the imaged photosensitive media with a heated platen, drum or belt, blowing heated air onto the media, immersing the media in a heated inert liquid and exposing the media to radiant energy of a wavelength to which the media is not photosensitive, e.g., infrared. Of these conventional techniques, the use of heated drums is particularly common.
A common photosensitive media useable in these imaging processes is known as a photothermographic media, such as film and paper. One photothermographic media has a binder, silver halide, organic salt of silver (or other deducible, light-insensitive silver source), and a reducing agent for the silver ion. In the trade, these photothermographic media are known as dry silver media, including dry silver film.
In order to precisely heat exposed photothermographic media, including film and paper, it has been found to be desirable to use electrically heated drums. In apparatus employing this technique, a cylindrical drum is heated to a temperature near the desired development temperature of the photothermographic media. The photothermographic media is held in close proximity to the heated drum as the drum is rotated about its logitudinal axis. When the temperature of the surface of the heated drum is known, the portion of the circumference around which the photothermographic media is held in close proximity is known and the rate of rotation of the drum is known, the development time and temperature of the thermographic media can be determined. Generally, these parameters are optimized for the particular photothermographic media utilized and, possibly, for the application in which the photothermographic media is employed.
U.S. Pat. No. 5,580,478, issued Dec. 3, 1996, inventors Tanamachi et al., discloses a temperature controlled, electrically heated drum for developing exposed photothermographic media. A cylindrical drum has a surface and is rotatable on an axis. An electrical heater is thermally coupled to the surface of the cylindrical drum. A temperature control mechanism, rotatably mounted in conjunction with the cylindrical drum and electrically coupled to the electrical heater, controls the temperature by controlling the flow of electricity to the electrical heater in response to control signals. A temperature sensor is thermally coupled to the surface of the cylindrical drum. A temperature sensor mechanism, rotatably mounted in conjunction with the cylindrical drum and electrically coupled to the temperature sensor, senses the temperature of the surface of the cylindrical drum and produces temperature signals indicative thereof. A microprocessor, non-rotatably mounted with respect to the cylindrical drum, controls the temperature of the electrically heated drum by generating the control signals in response to the temperature signals. An optical mechanism, coupled to the temperature control means, the temperature sensor means and the microprocessor means, optically couples the temperature signals from the rotating temperature sensor means to the non-rotating microprocessor means and optically couples the control signals from the non-rotating microprocessor means to the rotating temperature control means.
Separate electrical resistance heaters heat a central heat zone and contiguous edge zones. Temperature control of the electrical heaters is obtained through duty cycle modulation. Solid state relays in the power circuit to the electrical heaters are turned on and off with zero crossing triggering.
Although this technique is useful for the purpose for which it was intended, new flicker requirements of regulatory authorities in Europe (EC 65000-3-3) make this control technique unacceptable.
It is therefore desirable to provide a temperature control system for electrical resistor heaters that satisfy the new flicker requirements.
According to the present invention, there is provided a solution to the problems discussed above.
According to a feature of the present invention, there is provided a control system for reducing flicker in an electrical resistance heater comprising a source of AC (alternating current) current for supplying AC current to an electrical resistance heater, a bidirectional solid state switching device connected between said source and said electrical resistance heater; and a control circuit for controlling said bidirectional solid state switching device to supply a varying, phase controlled duty cycle of current to said heater which effectively ramps heater power up and down in response to a binary control signal which randomly turns on said switching device.
The invention has the following advantages.
1. New flicker requirements of a European agency are met without any internal software changes to the temperature control algorithms and with only minor changes to the circuit board.
2. The control technique is simple, cost efficient and effective.